happened to me too

I gave it the most ridiculous name i could and i still gives me the same error. I think something funny is going on.

I'll see if i can remote another computer in a different state and see if that works. However i'll be doing so on the assumption that A: it's not some bug evolved with my account B: you od not have to have an account to upload.

image dimensions?

When I encountered this problem a few months back, it was because I had forgotten to resize my banner to the standard 460 pixel width, 215 pixel height of standard Steam banners. It seems like the board rejects images that don't match those dimensions. I don't know if this applies to/helps you with your problem, though.